1. Field of the Invention
The present invention relates to an original illumination device with an automatic illumination control. Particularly, the present invention relates to an original illumination device used in a copying machine or the like, having automatic illumination control including a light receiving element for detecting reflected light from an original illuminated by a lamp. The automatic control is capable of checking any abnormal condition of the lamp and providing a power supply circuit to the lamp through the output of said light receiving element.
2. Description of the Prior Art
In an electrophotographic copying machine, an original document is illuminated by a lamp and reflected light of an image reflected from the original is directed to an uniformly charged photosensitive medium to form an electrostatic image corresponding to the original. The electrostatic image is treated in a known electrophotographic process to obtain a copy.
The density of the background areas of originals to be copied by such an electrophotographic copying machine varies widely, so that clear copies are not always obtained. For example, if an original having a darkish background, such as a newspaper, is illuminated by the same amount of light as is used for an original having a light background, the background of the copy thereof is fogged. A conventional copying machine is provided with a manual density control mechanism for manually changing the copy density in order to obtain clear copies. However, such manual density control mechanism required a trial and error operation, namely, one must take a trial copy and adjust the density according to the condition of the copy in order to obtain a proper copy.
In order to eliminate such troublesome operation, recently a mechanism for detecting the density of the original and for automatically changing the density of the copy according to the detected value has been proposed. For example, automatic copy density control mechanisms are disclosed in U.S. Pat. Nos. 4,153,364 and 4,200,391. In these automatic copy density control mechanisms, the density of the document is optically measured and the amount of light of the lamp or the bias voltage of the developing unit is controlled according to the measurement.
Meanwhile, the lamp utilized in the above described original illumination device is used under severe condition, for example, it is switched on and off intermittently at intervals of several seconds, or it is reciprocated during scanning of the original, so that the lamp often breaks. Since relatively high voltage such as 100 V or 200 V is applied to the lamp, when the lamp is lighted continuously even though where switching off of the lamp is necessary, it becomes extremely hot and may damage units around the lamp. Such being the circumstances, a copying machine provided with a lamp breakage detecting circuit or a monitoring circuit for abnormal lighting of the lamp has been proposed. For example, U.S. Pat. No. 4,272,183 discloses a mechanism for detecting lamp breakage, especially the breakage of an eraser lamp, in which light from a light radiating element connected in series with the eraser lamp is detected by a light receiving element and, in the case where no light from the light radiating element is detected, it is regarded as a lamp breakage and the operation of at least a charger will be stopped.
However, the separate provision of such a lamp breakage detecting circuit or a monitoring circuit for abnormal lighting of the lamp results in an increased cost, more complex structure or more complex control.
Each of the above described problems is similarly encountered not only in a copying machine but also in a reader printer for processing the light image of a microfilm, or in an image reader for optically reading a light image of an original illuminated by a lamp for outputting a digital output.